There are plural communication systems in mobile communication represented by a cellular phone. For example, in Europe, in addition to GSM widely used as a second generation for wireless communication system and EDGE in which data communication rate of GSM is improved, there is W-CDMA as a third generation for wireless communication system brought into operation in recent years. In North America, in addition to DCS and PCS as the second generation for wireless communication system, cdma-1x as the third generation for wireless communication system is spread. GSM is an abbreviation of Global System for Mobile Communication. EDGE is an abbreviation of Enhanced Data rate for GSM Evolution. W-CDMA is an abbreviation of Wide-band Code Division Multiple Access. DCS is an abbreviation of Digital Cellular System. PCS is an abbreviation of Personal Communication System. Cdma-1x is an abbreviation of Code Division Multiple Access 1x.
A mode of a radio frequency power amplifier included in a cellular phone terminal is a saturation mode in GSM in a basic mode in which only phase modulation is used and, is a linear mode at a bias point several dB backed off from a saturation mode point of GSM in EDGE, in which amplitude modulation is used together with phase modulation. The mode of the radio frequency power amplifier is also a linear mode in W-CDMA and cdma-1x in which amplitude modulation is used together with phase modulation.
In a radio frequency circuit block of a cellular phone terminal corresponding to GSM and EDGE, an antenna switch is arranged between a radio frequency power amplifier and an antenna. The antenna switch executes a function of switching a transmission slot and a reception slot of a TDMA (Time Division Multiple Access) system.
In a radio frequency circuit block of a cellular phone terminal corresponding to W-CDMA and cdma-1x, a duplexer is arranged between a radio frequency power amplifier and an antenna. The duplexer executes a function of processing transmission of a RF transmitting signal at a low RF frequency and reception of a RF reception signal at a high RF frequency in CDMA (code division multiple access) system in parallel. Moreover, in W-CDMA, cdma-1x, and the like, an isolator is arranged between the radio frequency power amplifier and the duplexer to prevent load mismatching in the antenna from affecting the radio frequency power amplifier. However, since it is difficult to integrate the isolator in a structure in which the radio frequency power amplifier is manufactured, the isolator is a large and expensive components.
In Hikaru Ikeda et al, “A Low Distortion and High Efficiency Parallel-Operation Power Amplifier Combined in Different Phases in Wide Range of Load Impedances”, 1996 IEEE MTT-S Digest, pp. 535-538 (hereinafter, Non-Patent Document 1), a parallel power amplifier that realizes low distortion and high efficiency with load impedance in a wide range without using an isolator is described. This parallel power amplifier has plural amplifying paths. An input signal of one input terminal is supplied to inputs of the plural amplifying paths by a hybrid divider. The respective amplifying paths include amplifiers and phase shifters. Phase shifters are arranged on the plural amplifying paths such that phases of operation of the amplifiers are different in the plural amplifying paths. Plural outputs of the plural amplifying paths are combined into a single output by a hybrid combiner. In Non-Patent Document 1, it is described that distortion equal to or lower than −45 dBc, efficiency equal to or higher than 45%, and a gain equal to or higher than 9.8 dB are obtained at a VSWR equal to or lower than 3:1 equivalent to a reflection coefficient Γ of 0.5. VSWR is an abbreviation of Voltage Standing-Wave Ratio. The VSWR is obtained from VSWR=(1+Γ)/(1−Γ) according to the reflection coefficient Γ.
In U.S. Pat. No. 6,954,623 (hereinafter, Patent Document 1), a balanced amplifier similar to the parallel power amplifier described in Non-Patent Document 1 is described. This balanced amplifier has plural amplifying paths. An input signal of one input terminal is supplied to inputs of the plural amplifying paths by a power divider. First phase shift components are arranged in the inputs of the respective amplifying paths and second phase shift components are arranged in outputs of the respective amplifying paths. An impedance transformer is connected to the second phase shift components. A power combiner combines plural outputs of the plural amplifying paths into one output. In Patent Document 1, it is described that, even if mismatching of impedance between an output of the power amplifier and the antenna occurs, an ACPR of the entire balanced amplifier is satisfactory. This is because impedance transformation of the power amplifier of one of the two amplifying paths changes to inductive rotation on a Smith chart and impedance transformation of the other power amplifier changes to capacitive rotation on the Smith chart. As a result, when one impedance is high impedance, the other impedance is low impedance. Thus, it is possible to correct distortion of a combined signal. ACPR is an abbreviation of Adjacent Channel Leakage Power Ratio. In Patent Document 1, it is described that a part of amplified transmitting signal energy of an output of the balanced amplifier is supplied to a power control block and, on the other hand, a D/A converter of a base band sub-system supplies a reference voltage as a power level signal to this power control block. This power control block is an output power control block of a closed loop that controls output power of the balanced amplifier. However, in Patent Document 1, there is no description about a specific structure of a circuit for supplying a part of the amplified transmitting signal energy of the output of the balanced amplifier to the power control block.
On the other hand, as another tendency concerning a radio frequency circuit structure in a cellular phone terminal, an output power detection circuit is built in a radio frequency power amplifier module including a radio frequency power amplifier. For example, in Jelena Madic et al, “Accurate Power Control Technique or Handset PA Modules with Integrated Directional Couplers”, 2003 IEEE Radio Frequency Integrated Circuits Symposium, pp. 715-718 (hereinafter, Non-Patent Document 2), it is described that a directional coupler that detects power generated by the power amplifier is integrated in a power amplifier module together with the power amplifier. A main path of the directional coupler is connected between the output of the power amplifier and the antenna. A coupled path of the directional coupler is connected between a termination resistor and an input of a power level control block. The directional coupler can detect a detected voltage of a vector sum of a combined voltage from a forward signal generated by the power amplifier and a combined voltage from a reverse signal reflected by a load.